Process for forming silicon oxide material

ABSTRACT

A thin layer of silicon oxide is formed by cyclic introduction of a silicon-containing precursor gas and an oxidizing gas separated by an intervening purge step. The resulting thin oxide layer enables subsequent conventional CVD of oxide to produce a more uniform deposited oxide layer over nonhomogenous surfaces, for example the silicon nitride mask/thermal oxide liner surfaces created during fabrication of shallow trench isolation structures.

BACKGROUND OF THE INVENTION

Silicon oxide is a dielectric material that is widely employed in thefabrication of semiconductor devices. Silicon oxide may be formed in anumber of ways. In one approach, silicon oxide may be thermally grownfrom an underlying silicon material through exposure to oxidizingagents.

Alternatively, silicon oxide may be formed through the process ofchemical vapor deposition (CVD). One example of such a CVD reactioninvolves the reaction of ozone (O₃) and tetraethylorthosilaneSi(OCH₂CH₃)₄ (TEOS) gases at elevated temperatures, resulting in thedeposition of silicon oxide as a solid material. In a typicalconventional TEOS O₃—O₂ CVD process for forming silicon oxide,silicon-containing precursor materials and oxidant are flowed into thedeposition chamber simultaneous with the application of heat. As aresult of these conditions, silicon oxide layers are rapidly formed.

One particularly important use for silicon oxide is in the formation ofshallow trench isolation (STI) structures between active devices of anintegrated circuit. FIG. 1A shows a cross-sectional view of the typicalstarting point for formation of an STI structure. Mask 102 comprisingsilicon nitride layer 104 overlying pad oxide layer 105 is patternedover silicon substrate 106. Silicon in unmasked regions 108 is removedto form trenches 110. Silicon sidewalls 112 of trenches 110 are thenexposed to an oxidizing ambient to form thermal oxide trench liner 114.

As shown in FIG. 1B, conventionally the shallow trench isolationstructure is formed by depositing silicon oxide over the entire surface,including over mask 102 and within trench 110. However, the thermallygrown oxide 114 provides a relatively inactive surface that results inlower rates of oxide deposition within trench 110. The higher rate ofdeposition of oxide over silicon nitride layer 104 of the mask 102 maycause greater accumulation of oxide material outside of trench 110,resulting in the possible formation of voids 116 within trench 110.Voids 116 can degrade the dielectric properties of the STI structurethat is ultimately formed.

Accordingly, new and improved processes for forming uniform, highquality layers of silicon oxide are valuable.

SUMMARY OF THE INVENTION

Embodiments in accordance with the present invention provide a thinlayer of silicon oxide formed by repeated cyclic introduction ofsilicon-containing and oxidizing reactant gases to a deposition chamber.The resulting thin oxide layer enables subsequent conventional oxide CVDto create a more uniformly deposited oxide layer over nonhomogenoussurfaces, for example the silicon nitride mask/thermal oxide linersurfaces created during STI formation.

An embodiment of a method of forming a thin silicon oxide layer over asubstrate disposed in a substrate processing chamber comprisesintroducing tetraethylorthosilane into the processing chamber. Thetetraethylorthosilane is purged from the processing chamber, and thenozone is introduced into the processing chamber after purging of thetetraethylorthosilane. The ozone is then purged from the processingchamber. This cycle of steps may be repeated to create additionalsilicon oxide material.

An embodiment of a method for treating a surface to receive chemicalvapor deposited silicon oxide in accordance with the present inventioncomprises exposing the surface to a silicon-containing precursor gas ina processing chamber, and purging the silicon-containing precursor gasfrom the processing chamber. An oxidant is introduced into theprocessing chamber after purging the silicon-containing precursor gas.The oxidant is purged from the processing chamber, such that a thinlayer of oxide is formed over the surface to serve as a basis forsubsequent uniform chemical vapor deposition of silicon oxide.

An embodiment of a method of forming a shallow trench isolationstructure on a silicon substrate having a plurality of trenches etchedtherein to define isolation regions and a plurality of masked regions onan upper surface of said substrate positioned between said isolationregions, said method comprising exposing the substrate to an oxidizingambient to create a thermal oxide layer within the trench. A layer ofsilicon oxide is formed over the thermal oxide layer by alternating (i)introducing to the chamber a first gas consisting of one of asilicon-containing precursor gas and an oxidant, (ii) purging the firstgas from the chamber, (iii) introducing to the chamber a second gasconsisting of the other of the silicon-containing precursor gas and theoxidant, and (iv) purging the second gas from the chamber. Steps(i)-(iv) are repeated until a desired thickness of the silicon oxidelayer is achieved; and then the trenches are filled with chemical vapordeposited silicon oxide material.

These and other embodiments of the present invention, as well as itsfeatures and some potential advantages are described in more detail inconjunction with the text below and attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B show simplified cross-sectional views of the formation ofoxide-filled trenches utilizing a conventional CVD process.

FIG. 2 shows a timing diagram of one embodiment of a silicon oxidedeposition process in accordance with the present invention.

FIG. 3A plots deposited film thickness versus silicon substratetemperature for silicon oxide deposition processes utilizing TEOS/O₃ andO₃ only.

FIG. 3B plots deposited film thickness versus number of gas exposurecycles for silicon oxide deposition processes utilizing TEOS/O₃ and O₃only.

FIGS. 4A-4C show simplified cross-sectional views of the formation ofoxide-filled trenches utilizing a process in accordance with anembodiment of the present invention.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

A thin layer of silicon oxide is formed by repeated cyclic introductionof silicon-containing and oxidizing reactant gases to a depositionchamber separated by an intervening purge step. The resulting thin oxidelayer enables subsequent conventional oxide CVD performed in the samechamber to create a uniform deposited oxide layer over nonhomogenoussurfaces, for example the silicon nitride mask/thermal oxide linersurfaces present during STI formation.

Control over the formation of thin oxide layers in accordance withembodiments of the present invention is accomplished by repeated cyclicintroduction of silicon-containing and oxidizing precursor gasesseparated by a purge step. In an embodiment of a method for formingsilicon oxide in accordance with the present invention, introduction ofa TEOS/carrier gas is followed by a first purge. Next, an oxidant in theform of an oxygen/ozone mixture is introduced into the chamber and thenremoved by a second purge step to form a very thin layer of siliconoxide. Repetition of this cycle of steps affords precise control overthe thickness and quality of the oxide layer ultimately formed.

One chemical system that is useful for formation of thin oxide layers inaccordance with an embodiment of the present invention utilizestetraethylorthosilane (TEOS) as the silicon-containing precursormaterial and ozone as the oxidant.

(A) SiOH*+Si(OCH₂CH₃)₄→SiOSi(OEt)₃*+EtOH (@<10 torr, <0.5-2 sec)

(B) purge TEOS

(C) SiOEt*+O₃→SiOH*+SiOSi*+CO+H₂O (@<10 torr, <0.5-2 sec)

(D) purge ozone

FIG. 2 shows a timing diagram of one embodiment of a silicon oxidedeposition process in accordance with the present invention. At firsttime T₁, a mixture of TEOS and a carrier gas are introduced into thechamber. At a second time T₂ about 0.5-2 seconds after T₁, theTEOS/carrier gas flow is halted.

At a third time T₃, an inert gas (such as He, Ar, N₂, or combinationsthereof) is introduced to the chamber to purge any remainingTEOS/carrier gas. At a fourth time T₄, the flow of the purge gas ishalted.

At a fifth time T₅, ozone is introduced into the chamber. At a sixthtime T₆ about 0.5-2 seconds after T₅, the flow of ozone into the chamberis halted.

At a seventh time T₇, an inert gas is introduced to the chamber to purgeany remaining ozone. At an eighth time T₈, the flow of the purge gas isagain halted, setting the stage for another TEOS/purge/ozone cycle todeposit oxide.

FIG. 3A plots deposited film thickness versus substrate temperature forcyclic silicon oxide deposition processes utilizing TEOS/O₃ and O₃ only.FIG. 3A shows that for the TEOS/O₃ reaction conditions, above oxideformation occurs above 420° C. and the rate of deposition increases withincreasing substrate temperature. FIG. 3A also shows that oxideformation in accordance with embodiments of the present invention isdistinct from the oxide formation attributable to thermal oxidation ofthe silicon surface occurring above 540° C.

FIG. 3B plots deposited film thickness versus number of gas exposurecycles for silicon oxide deposition processes utilizing TEOS/O₃ and O₃only. For the TEOS/O₃ deposition conditions, FIG. 3B shows a linearcorrelation between the number of cycles and the thickness of filmgrowth. By contrast, little or no film growth occurs in the pure O₃ambient even after over a hundred cycles have taken place.

The TEOS/O₃ oxide deposition conditions just described likely do notresult in formation of silicon oxide one monolayer at a time.Specifically, TABLE A presents surface roughness data revealed by atomicforce microscopy (AFM) of a silicon oxide layer formed on the center andedge of a wafer utilizing one embodiment of the present invention.

TABLE A WAFER ROUGHNESS (nm) SAMPLE LOCATION RMS R_(a) R_(max) Bare Sicenter 0.18 0.14 2.39 conv. CVD center 0.26 0.21 2.31 TEOS + O₃ center0.17 0.13 2.37 After 20 cycles TEOS + O₃ center 0.24 0.18 3.27 After 80cycles TEOS + O₃ center 0.59 0.47 5.35 After 160 cycles O₃ only center0.18 0.14 2.83 After 20 cycles O₃ only center 0.20 0.14 3.30 After 80cycles O₃ only center 0.18 0.14 2.47 After 160 cycles Bare Si edge 0.190.15 3.23 conv. CVD edge 0.25 0.20 2.03 TEOS + O₃ edge 0.17 0.13 4.76After 20 cycles TEOS + O₃ edge 0.31 0.23 4.44 After 80 cycles TEOS + O₃edge 0.72 0.57 6.67 After 160 cycles O₃ only edge 0.18 0.14 2.44 After20 cycles O₃ only edge 0.18 0.14 1.72 After 80 cycles O₃ only edge 0.210.17 3.46 After 160 cycles

The AFM data of TABLE A shows that as the oxide film grows from 20 to100 Å (20 cycles to 160 cycles), the average roughness (R_(a)) of thesilicon oxide film increases from about 0.13 to about 0.47 nm, a surfaceroughness comparable with that resulting from formation by conventionalCVD processes. This increased surface roughness tends to indicate thatcyclic silicon oxide deposition in accordance with embodiments of thepresent invention does not occur precisely one atomic layer at a time.

Embodiments of processes for depositing a layer of silicon oxidematerial in accordance with the present invention have many potentialapplications. One application is in the formation of STI structures, asillustrated below in FIGS. 4A-4C.

FIG. 4A shows a cross-sectional view of the typical starting point offormation of an STI structure. Mask 402 comprising silicon nitride layer404 overlying pad oxide layer 405 is patterned over silicon substrate406. Silicon in unmasked regions 408 is removed to form trenches 410having a depth Y that may typically be between about 0.3-0.4 μm.Sidewalls 412 of trenches 410 are then exposed to an oxidizing ambientto form thermal oxide trench liner 414.

As previously shown and described in conjunction with FIG. 1B, inconventional processes the shallow trench isolation structure is formedby depositing silicon oxide over this entire nonhomogenous surface,including over mask 402 and within trench 410. However, the thermallygrown oxide liner layer is relatively inactive, and hence the rate ofdeposition of oxide over silicon nitride layer of the mask may causegreater accumulation of material outside of the trench, resulting in thepossible formation of gaps within the trench.

Accordingly, a thin layer of oxide may first be formed over the entiresurface to provide a basis for subsequent uniform CVD of oxide. As shownin FIG. 4B, oxide layer 418 is formed in accordance with embodiments ofthe present invention through cyclic introduction of silicon-containingand oxidizing gases separated by purge steps. The thickness of layer 418formed in FIG. 4B can be precisely controlled so that there is no riskof the formation of gaps within the trenches. The thickness of layer 418may range from about 10-100 Å, and is most preferably between about20-30 Å.

As shown in FIG. 4C, thin oxide layer 418 provides a uniform oxidesurface to serve as a template for subsequent rapid formation of oxide415 by conventional CVD. Examples of conventional CVD silicon oxideformation processes include but are not limited to the mixing ofoxidants and silicon-containing precursor gases at elevatedtemperatures, reduced pressures, or in the presence of plasma. Examplesof oxidants include but are not limited to oxygen, ozone, steam, andhydrogen peroxide. Examples of silicon-containing precursor gasesinclude but are not limited to TEOS, silane, SiCl₄, Si(NCO)₄, andCH₃OSi(NCO)₃.

As a result of the presence of the thin oxide starting surface 418,rates of oxide formation by conventional CVD techniques are similar bothinside and outside of trench 410, and thus little or no gap is created.

The above description is illustrative and not restrictive, and as suchthe process parameters and applications listed above should not belimiting to the claims as described herein. For example, while theinvention is illustrated above with reference to one particularembodiment, one of ordinary skill in the art would recognize that thepresent invention is not limited to this particular example.

Thus while the above discussion has described a cyclic thin oxideformation process in which the silicon-containing precursor gas isintroduced first, embodiments of the present invention are not limitedto such an example. In accordance with alternative embodiments of thepresent invention, the surface upon which the thin oxide is sought to beformed may be exposed to the oxidizing gas as a first step prior tointroduction of the silicon-containing precursor gas.

Moreover, the present invention has been described so far in connectionwith formation of the thin oxide layer followed by performance of theconventional CVD process in the same chamber utilizing the samereactants, thereby obviating the need for a wafer transfer step andincreasing throughput. However, formation of the thin oxide layer in thesame chamber utilizing the same reactants as the subsequent conventionalCVD step is not required by the present invention. In accordance withalternative embodiments of the present invention, a cyclic thin oxideformation process and a subsequent CVD process utilizing the same ordifferent reactants could be performed in different processing chambers.

In addition, while the invention has been described so far in connectionwith formation of the thin silicon oxide layer through the use of ozoneand TEOS, the invention is not limited to this particular embodiment.Other reactants could be employed to create a thin, uniform oxide layeras a starting point for a subsequent, more rapid and uniformconventional CVD deposition step, and the resulting method or apparatuswould fall within the scope of the present invention.

For example, a two-stage oxide formation process involving differentreaction systems could be utilized to create a thin oxide layer prior toconventional oxide CVD in accordance with the present invention. Onealternative reaction system may employ steam as the oxidant and SiCl₄ asthe silicon-containing precursor. Another alternative reaction systemmay employ steam as the oxidant and Si(NCO)₄ as the silicon-containingprecursor. Still another example of an alternative reaction system mayemploy hydrogen peroxide as the oxidant and CH₃OSi(NCO)₃ as thesilicon-containing precursor.

As with the TEOS/O₃ reactant system described above, a thin siliconoxide layer produced by cycling the two stages of these reaction systemswould enhance uniformity of oxide that is subsequently formed byconventional CVD processes.

Given the above detailed description of the present invention and thevariety of embodiments described therein, these equivalents andalternatives along with the understood obvious changes and modificationsare intended to be included within the scope of the present invention.

1. A method of forming a shallow trench isolation structure on a siliconsubstrate having a plurality of trenches etched therein to defineisolation regions and a plurality of masked regions on an upper surfaceof said substrate positioned between said isolation regions, said methodcomprising: exposing the substrate to an oxidizing ambient to create athermal oxide layer within the trench; forming a layer of silicon oxideover the thermal oxide layer by alternating (i) introducing to thechamber a first gas consisting of one of a silicon-containing precursorgas and an oxidant, (ii) purging the first gas from the chamber, (iii)introducing to the chamber a second gas consisting of the other of thesilicon-containing precursor gas and the oxidant, (iv) purging thesecond gas from the chamber, and (v) repeating steps (i)-(iv) until adesired thickness of the silicon oxide layer is achieved; and fillingthe trenches with chemical vapor deposited silicon oxide material. 2.The method of claim 1 wherein the silicon-containing precursor gascomprises tetraethylorthosilane (TEOS) and the oxidant comprises ozone.3. The method of claim 1 wherein the silicon-containing precursor gascomprises SiCl₄ and the oxidant comprises steam (H₂O).
 4. The method ofclaim 1 wherein the silicon-containing precursor gas comprises Si(NCO)₄and the oxidant comprises steam (H₂O).
 5. The method of claim 1 whereinthe silicon-containing precursor gas comprises CH₃OSi(NCO)₄ and theoxidant comprises hydrogen peroxide (H₂O₂).